Mutations in enzymes involved in protein O-mannosyl glycosylation produce a group of devastating diseases characterized by congenital muscular dystrophy (CMD), type II lissencephaly, and eye abnormalities. Previous studies using animal models have demonstrated that hypoglycosylation of a-dystroglycan critically contributes to the pathogenesis of these disorders. A growing body of evidence, however, suggests that other substrates for O-mannosyl glycosylation likely contribute to particular brain abnormalities found both in animal models and in patients with CMDs. Unfortunately, previous studies have failed to identify the additional neural substrates of O-mannosylation that contribute to these brain pathologies. Using an animal model of CMD in which the glycosyltransferase POMGnT1 is knocked out we identified RPTP? as another important substrate for O-mannosyl glycosylation in the brain. Receptor tyrosine phosphatase zeta/beta is a receptor phosphatase that is found predominately in the central nervous system and is highly expressed during key stages of neural development. Both membrane-bound receptor and secreted variants of the protein are expressed in the brain and are high affinity ligands for a number of developmentally important adhesion molecules, growth factors and extracellular matrix proteins. We hypothesize that the hypoglycosylation of RPTP6 due to disrupted O-mannosylation alters the interactions of RPTP? with these key ligands and contributes to abnormal brain development. Consistent with this hypothesis we found that cortical neuron development in cultured cells from POMGnT1 knockouts is abnormal when they are plated on a subset of known RPTP? ligands. In this proposal we investigate the hypothesis that the hypoglycosylaton of RPTP? in O-mannosyl glycosylation mutant animals alters its ligand-binding characteristics thereby leading to aberrant cellular interactions and abnormal brain development. The proposal is focused on the following 3 specific aims: 1) To determine whether altered glycosylation of RPTP? modulates its ligand binding characteristics. 2) To determine whether disrupted glycosylation of RPTP? alter cell development. And 3) To identify the 1-dystroglycan-independent neural abnormalities in animal models of CMDs and to determine if these abnormalities are due to altered RPTP?/phosphacan glycosylation. To explore these questions, we will utilize a variety of genetic mouse model systems in which we will disrupt O-mannosyl glycosylation, a-dystrglycan expression or RPTP?expression. Studies will be conducted both in vitro and in vivo to determine how O-mannosyl glycans modulate RPTP? function and if this contributes to brain abnormalities found in animal models of CMDs. These studies will determine if RPTP? is an important contributor to CMD pathogenesis and will provide important insights into the pathomechanisms of the neural phenotypes in these disorders.